In the silicon industry, research into reducing feature size and registration is approximately $100 million/year, and these fabrication strategies are increasingly expensive and inaccessible to researchers. As an alternative, scanning probe lithography (SPL) has become increasingly popular in the fabrication of nanoscale structures as a consequence of its cost advantage, capability to achieve a high resolution, alignment accuracy exceeding most of the existing technologies, as well as reliability. In particular, molecular printing, defined as the deposition of molecules directly onto a surface with at least one feature dimension on the molecular scale, via tip-based scanning probe offers an alternative approach to creating nanoscale features with high resolution and low-cost. However, increasing the throughput, while maintaining the advantages of SPL, has been a significant challenge.
Recent advances in molecular printing have led to important advances in biotechnology, materials science, and electronics: leading to devices such as lab-on-a-chip assays, genetic and proteomic arrays, and novel memory device architectures, however, these applications are in their nascent stages because of the rapidly developing nature of molecular printing technologies and the lack of reliable strategies for increasing the throughput or reducing the feature sizes produced by these methods. Among the most familiar molecular printing methods are soft lithography and dip-pen nanolithography. Soft lithography refers to a class of molecular printing methods, including its most widely used incarnation microcontact printing (μCP), in which a patterned elastomeric stamp is pressed against a surface, leaving a pattern of molecules that mirrors the topography of the stamp. An advantage of soft lithography is the ability to pattern large areas, but the mechanical properties of the stamp limit the patterns that can be made by μCP: typically the feature sizes must be greater than 200 nm in diameter, and the patterns are limited by lateral collapse of the stamp when features are too close together and roof collapse when features are separated by too great a distance. SPL, in which a sharp stylus is attached to piezoelectric actuators is utilized to pattern a surface, has been widely investigated in the context of nanopatterning. Because of the mechanical strength of the hard AFM tips, techniques that involve the transfer of energy to a surface, such as scratching, etching, and oxidizing surfaces are well-established. Dip-pen nanolithography (DPN) is a scanning-probe based molecular printing method that uses the tip of an atomic force microscope (AFM) coated with an ink as a pen to transfer the ink directly onto a surface, and because the aqueous meniscus that forms between the tip and the surface serves as a conduit for ink transfer, there is a linear relation between the dwell time of the tip on the surface and the feature area. Because of the small tip diameter, lines with a width as small as 15 nm can be written on a single crystal Au surface, and as a result of the piezoelectric actuators that control the movement of the tip, arbitrary patterns with nanometer registration between features are an important capability of SPL. DPN has been used to write nanoscale patterns of nanoparticles, DNA, proteins, and various small molecules, resulting in applications such as gene chips, assays for the HIV-1 p24 antigen, bio-screening devices, gas sensors, and photomasks, but the slow writing speed of single-pen DPN limits it to prototyping applications. Massively parallel pen arrays of containing as many as 55,000 pens immobilized onto cantilevers have been microfabricated, but these arrays are expensive and difficult to use, so increasing the throughput of scanning probe lithographies remains an active area of investigation.
The issues of throughput for tip-based molecular printing methods have been addressed recently by the advent of Polymer Pen Lithography (PPL), a new tip-based molecular printing method that combines elements of DPN and soft lithography. PPL utilizes pen arrays containing as many as 107 pyramidal tips, and these arrays are mounted onto the piezoelectric actuators of an AFM to create arbitrary patterns with nanoscale feature diameter and registration. PPL maintains the time-dependent feature size control characteristic of DPN, but also can use force-dependent feature size control due to the elastomeric tip deformation upon application of force. With the current state of PPL, a digitized pattern can be printed with sub-100-nm control over feature diameter and position, which enables the production of features with diameters ranging from 80 nm to greater than 10 μm in a single writing operation.
There is a need to develop a new massively parallel, tip-based molecular printing method that is convenient and can create sub-micron features beyond the limit of PPL, while maintaining the low-cost and ease of use characteristic of PPL.